Polymer reinforced materials for inkjet based 3d printing

ABSTRACT

The present disclosure relates to reinforcing photopolymer resins and uses thereof, e.g., in inkjet 3D printing.

RELATED APPLICATIONS

The application claims the benefit of the Oct. 17, 2018 priority of U.S.Provisional Application 62/746,818, the contents of which are hereinincorporated by reference.

FIELD OF THE INVENTION

The invention relates generally to 3D inkjet printing and morespecifically to the printing of reinforcing material in using a 3Dinkjet printer.

BACKGROUND OF THE INVENTION

Additive manufacturing allows objects to be fabricated via selectiveaddition of material. A typical additive manufacturing process works byslicing a digital model (for example, represented using an STL file)into layers. Data representing these layers is then sent to afabrication apparatus. The fabrication apparatus then builds an objectby depositing one layer at a time, starting with a bottom layer andending with a top layer. Additive manufacturing is rapidly gainingpopularity in a variety of markets including automotive, aerospace,medical devices, pharmaceuticals, and industrial tooling.

The growth of additive manufacturing processes has led to thecommercialization of various species of such processes. These includeextrusion processes, such as fused deposition modeling® (FDM®) lightpolymerization processes, such as stereolithography (SLA) andmultijet/polyjet powder bed fusion processes, such as selective lasersintering (SLS) or binder jetting, and lamination processes, such aslaminated object manufacturing (LOM).

Nevertheless, despite its growth, additive manufacturing haslimitations. Among these limitations are the constraints in thematerials that can be used in conjunction with such processes. Onlylimited types of materials can be used. The performance of thesematerials limits the efficiency of the manufacturing process and thequality of the manufactured object.

One example of an additive manufacturing process relies on 3D printingusing an inkjet printer. In this example, printheads build an object bydepositing droplets of liquid ink. These printheads are typicallymounted on a gantry system to allow deposition of ink in differentlocations of a build volume. A build platform may also move with respectto the printheads, which may be stationary. The liquid ink is thensolidified, for example by using UV or visible-light radiation.

Because of its high resolution, its high throughput, and its ability toprint multiple materials, an inkjet printers are particularly useful forbuilding prototypes. These printers typically rely on low-viscosityacrylates. The resulting structures are useful as prototypes but lackthe durability and strength that would be desired in an actual endproduct.

In some systems, multiple printheads build objects with multiple basematerials. For example, materials that have different optical,mechanical, thermal, or electromagnetic properties can be used. Thesematerials can be combined to achieve composite materials with a widerange of properties.

An inkjet printer that carries out additive manufacturing typically hasa subsystem for curing the ink. These subsystems typically rely on UVcuring.

In a UV-curing unit, UV radiation solidifies inks via photo-initiationof a polymerization reaction. UV radiation can be supplied by a varietyof different mechanisms, such as arrays of LEDs and mercury or xenon arclamps. UV-curing is typically applied after each printed layer or afterdepositing each material within a layer. The UV-curing unit can be fixedwith respect to the printer or it can move independently with respect tothe object.

Some inkjet printers that carry out additive manufacturing combine bothUV and thermal curing to manufacture an object.

Inks suitable for inkjet printing often conform to certainspecifications. Of particular importance are the ink's viscosity and itssurface tension. Suitable viscosities are in the range of 10-15 cps atoperating conditions. Surface tension typically should be between 20-45mN/m.

An ink is preferably thermally stable. For example, it should notsolidify anywhere within the ink jet printer. In particular, it shouldremain liquid within the printhead, the ink container, and the feedingsystem.

It is also useful for the ink to have formulation stability. Inparticular, the different components of the ink should not separate fora reasonably long time.

Many inks include additives. These additives include colorants, such asdyes or pigments or the mixture of pigments and dyes. These colorantsare dispersed or dissolved in the ink. Some inks also includesurfactants adjust the surface tension of the ink. These surfactantspromote jetting or printing performance

Since different inks have different properties, it is necessary tooptimize various parameters based on these different properties. As oneexample, the process of inkjet printing requires causing the printheadto eject the ink. This is carried out by applying a waveform to theprint head. The shape and duration of this waveform depends on thenature of the ink to be ejected. Therefore, it is important to optimizethis waveform for different ink formulations.

SUMMARY OF THE INVENTION

The invention provides ways to reinforce the materials used in 3D inkjetprinting.

Upon being cured, an ink becomes a polymer matrix. It is possible toimprove the strength of such a matrix by adding a reinforcing fillermaterial. Examples of suitable filler materials include carbon black,silica, clay, glass fibers, and carbon nanotubes. It is particularlyuseful to add filler materials at high loading fractions, often 30-100phr, in order to adequately improve the strength of the cured polymermatrix.

Although adding a filler material provides reinforcement, the highloading fraction of filler that is required to achieve meaningfulstrengthening increases the resin viscosity. This can push viscositybeyond what is acceptable for use as an ink in a 3D printer.

The use of filler material also causes difficulty because the ink mustremain a homogenous liquid until it is cured. This requires that thefiller material disperse well and that it be stable over long periods oftime. This limits both the maximum amount of filler and the type offiller material that is usable with photopolymers.

Some 3D printing methods, particularly those that rely on inkjetprinting, have even stringent requirements that further limit theability to disperse filler. For example, an inkjet printer may encounterdifficulty if there exist particles in the ink that exceed a maximumparticle size.

One aspect of the invention is related to printing using an ink that isinitially liquid but that transforms, during curing, into a 3D matrixmaterial in which a constituent of the liquid ink forms a second phasewithin the 3D printable matrix material. This second phase adds strengthand support to the 3D printable matrix material. The result is thereforea reinforced matrix material that provides additional strength anddurability to objects that have been built using a 3D inkjet printingprocess.

In one embodiment, the invention relates to a method of reinforcingphotopolymer resins for improved tensile strength while maintaining theprocessability of matrix resins.

In another embodiment, the reinforcing agent is initially soluble orpartially soluble in the uncured matrix material resin but undergoes adecrease in solubility when the uncured matrix material resin is cured.

In yet another embodiment, the reinforcing agent initially is soluble orpartially soluble in the uncured matrix material resin but undergoes adecrease in solubility when the uncured matrix material resin undergoesa temperature change. This causes the reinforcing material toprecipitate from solution and to separate into a different phase. Thisforms domains of rigid material that serve to strengthen the polymermatrix.

In another aspect, the invention relates to a reinforced composition for3D ink printing comprising: a UV-curable matrix material and areinforcing agent that is at least partially soluble in the UV-curablematrix material.

Embodiments include those in which the UV-curable matrix materialincludes acrylates, thiol-enes, or combinations thereof.

In other embodiments, the reinforcing agent is selected based at leastin part on one or more properties of the UV-curable matrix material.

In yet other embodiments, the reinforcing agent has a higher glasstransition temperature than that of the UV-curable matrix material.

Further embodiments include those in which the reinforcing agent isnon-UV-curable and those in which the reinforcing agent does not reactwith the UV-curable matrix material.

Also among the embodiments are those in which the tensile strength ofthe cured reinforced composition is 30-300% higher than the UV-curablematrix material in the absence of any reinforcing agent and those inwhich the tensile strength of the cured reinforced composition dependsat least in part upon the loading of the reinforcing agent within thereinforced composition.

Further embodiments include those in which the content of reinforcingagent is less than 20 wt % in the overall formulation.

A variety of reinforcing agents can be used. These include, asrepresentative examples, polyvinylpyrrolidone and a poly(methylmethacrylate).

DESCRIPTION OF A PREFERRED EMBODIMENT

A composition of the 3D printable material includes a printable matrixmaterial, which is curable, and a reinforcing agent. The printablematrix material is typically a monomer that polymerizes under UVirradiation. The printable matrix material is generally, but not limitedto, acrylates, thiol-enes or combinations thereof.

The reinforcing agent is not UV-curable and is non-reactive with theprintable matrix material. The reinforcing agent is, however, at leastpartially soluble in the printable matrix material. The reinforcingagent and the printable matrix material may be mixed as liquids, or thereinforcing agent may be dissolved as a solid into the liquid printablematrix material to form a solution. The reinforcing agent has a higherglass transition temperature (Tg) than the printable matrix material. Inone embodiment, the reinforcing agent constitutes less than 20% of thematrix material-reinforcing solution by weight.

Although the reinforcing agent is at least partially soluble in theprintable matrix material, as the printable matrix material is curedunder UV irradiation or otherwise undergoes a temperature decrease, thereinforcing agent becomes less soluble and precipitates out of solutionforming another phase within the printable matrix material. This phaseforms reinforcing structures within the curing matrix material.

The principle may be understood by considering the followingnon-limiting examples.

EXAMPLE 1 Material: Acrylate Matrix Reinforced by a Vinylpyrrolidone

In this embodiment, IPUC101, an inventor-formulated elastomeric acrylatematerial, is reinforced by Polyvinylpyrrolidone (PVP) (Mw: 6000-150000).IPUC101 is a self-formulated (see Table 1), elastomeric acrylatematerial with a tensile strength of 3.4 MPa, elongation at break of160%, and Shore hardness of 35A. The reinforcing agent isPolyvinylpyrrolidone (PVP) K15, purchased from Tokyo Chemical IndustryCo. Ltd. The reinforcing agent PVP is a polymer known for having highpolarity. Because this formulation of IPUC101 includes almost 30%2-Hydroxyethyl acrylate (HEA), IPUC101 is also polar to some extent. TheHEA in the IPUC101 helps the PVP to dissolve or disperse into theuncured material. The reinforcing effect on IPUC101 is a function ofboth the composition of reinforcing agent and the concentration of thereinforcing agent. In this embodiment using PVP, the molecular weightwas shown to have a large effect on the reinforcement.

Preparation of PVP-Rein Forced IPUC101

In one embodiment, the desired amount of PVP and IPUC101 were dispensedinto a sealed amber bottle. The mixture was stirred at elevatedtemperature (for example 70° C.) until all solids were dissolved. Inanother embodiment, the PVP powder and IPUC101 were added into acontainer and mixed using a Flextek mixer (Flex-Tek Group, Greenwood,S.C., USA) at room temperature until the powder was uniformly dissolvedin the solution. Each resulting solution was either clear or slightlycloudy. The inks were stored at room temperature until use.

The reinforcing effect of adding PVP K15 to the IPUC101 formulation isshown in Table 2. The tensile strength and Shore hardness of thematerial increases as PVP K15 is added to the formulation. The tensilestrength of the IPUC101 material without PVP is 3.4 MPa, but increasesto 4.6, 7.4, and 9.6 MPa at loading fractions of 1.5%, 3%, and 5% PVPK15 respectively. The Shore A hardness similarly increases from 35 to38, 40, and 45 respectively. Notably, the elongation at break does notappear to change significantly. Also, of note is the degree ofreinforcement at low loading fraction. Typical fillers require 30-100phr for adequate reinforcement, whereas PVP K15 reinforcement more thandoubles the tensile strength at only 3% loading.

TABLE 1 IPUC101 Formulation Percentage Supplier Function Photomer 623029.98 IGM Resins Oligomer 2-Hydroxyethyl acrylate 29 TCI America MonomerSR440 20 Sartomer Monomer Genomer 1121 20 Rahn AG Monomer Omnirad 819 1IGM Resins Photo initiator MEHQ 0.02 Sigma Photo inhibitor Total 100

TABLE 2 With and Without PVP K15 Reinforcement: Mechanical PropertiesTensile Elongation at Hardness strength (MPa) break (%) (Shore A)IPUC101 3.4 160 35 IPUC101 − 1.5% K15 4.6 162 38 IPUC101 − 3% K15 7.4166 40 IPUC101 − 5% K15 9.6 160 45Table 3 below shows how viscosity of the acrylate resin IPUC101formulation as a function of temperature without any reinforcing filleras well as with varying amounts of reinforcing filler.

IPUC101 + IPUC101 + IPUC101 + IPUC101 1.5% K-15 3% K-15 5% K-15 30 C.30.70 cP 36.65 cP 43.17 cP 49.31 cP 40 C. 20.76 cP 24.66 cP 28.78 cP31.81 cP 50 C. 14.89 cP 17.46 cP 20.19 cP 22.83 cP 60 C. 11.30 cP 13.09cP 14.85 cP 16.58 cP 70 C.  8.48 cP  9.82 cP 11.24 cP 12.55 cP

EXAMPLE 2 Material: Acrylate Matrix Reinforced by a SecondVinylpyrrolidone

In this embodiment, the vinylpyrrolidone is Polyvinylpyrrolidone K12provided by BASF. The preparation of samples of IPUC101 reinforced withPVP K12 is the same with IPUC101 reinforced by PVP K15 (Mw: 4000-6000).The mechanical properties of PVP K12 reinforced IPUC101 is shown inTable 3. As shown in Table 3, with 5 wt % of loading, there is a 40% ofincrease in tensile strength. Although the tensile strength wasincreased, the increase is not as significant as when PVP K15 is thereinforcing agent.

TABLE 3 PVP K12 Reinforcement: Mechanical Properties Tensile Elongationat Hardness strength (MPa) break (%) (Shore A) IPUC101 3.4 160 35IPUC101 − 3% K12 3.7 156 40 IPUC101 − 5% K12 4.9 161 40

EXAMPLE 3 Material: Thiol-ene Matrix Reinforced by Poly(MethylMethacrylate) PMMA

In this embodiment, a new Thiol-ene matrix material, TE14, is aself-formulated (see table 4), elastomeric thiol-ene material with atensile strength of 1 MPa, elongation at break of 107%, and Shorehardness of 30 A. The reinforcing material PMMA (Mw: 120000) waspurchased from Sigma.

TABLE 4 TE14 Formulation Percentage Supplier Function Trimethylolpropanetris(3- 15.15 Sigma Monomer mercaptopropionate) 3,6-dioxa-1,8- 29.6 TCIAmerica Monomer octanedithiol Diallyl phthalate 53.7 TCI America MonomerVinylphosphonic acid 0.5 TCI America Photo inhibitor Omnirad 651 1 IGMResins Photo initiator Pyrogallol 0.05 TCI America Photo inhibitor Total100

Preparation of TE14 Reinforced by PMMA

In this embodiment, a desired amount of each of PMMA and TE14 wasdispensed into a sealed amber bottle. The mixture was stirred atelevated temperature (for example 70° C.) until all solids weredissolved. Alternatively, PMMA powder and TE14 were added into acontainer and mixed through a Flextek mixer at room temperature untilthe powder was dissolved in the solution. The resultant solutions wereclear. The composition was stored at room temperature until use.

The reinforcing effect of adding PMMA to the TE14 formulation is shownin Table 5. The tensile strength and Shore hardness of the materialincreases as PMMA is added to the formulation.

TABLE 5 Reinforcing Effect of Adding PMMA to the TE14 FormulationTensile Elongation at Hardness strength (MPa) break (%) (Shore A) TE141.0 107 30 TE14 − 3% PMMA 1.3 236 32 TE14 − 5% PMMA 1.7 184 35

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the disclosure. For example, various formsof the materials shown above may be used, with steps re-ordered, added,or removed. Accordingly, other implementations are within the scope ofthe following claims.

The examples presented herein are intended to illustrate potential andspecific implementations of the present disclosure. The examples areintended primarily for purposes of illustration of the invention forthose skilled in the art. No particular aspect or aspects of theexamples are necessarily intended to limit the scope of the presentinvention.

The figures and descriptions of the present invention have beensimplified to illustrate elements that are relevant for a clearunderstanding of the present invention, while eliminating, for purposesof clarity, other elements. Those of ordinary skill in the art mayrecognize, however, that these sorts of focused discussions would notfacilitate a better understanding of the present disclosure, andtherefore, a more detailed description of such elements is not providedherein.

Unless otherwise indicated, all numbers expressing lengths, widths,depths, or other dimensions and so forth used in the specification andclaims are to be understood in all instances as indicating both theexact values as shown and as being modified by the term “about.” As usedherein, the term “about” refers to a ±10% variation from the nominalvalue. Accordingly, unless indicated to the contrary, the numericalparameters set forth in the specification and attached claims areapproximations that may vary depending upon the desired propertiessought to be obtained. At the very least, and not as an attempt to limitthe application of the doctrine of equivalents to the scope of theclaims, each numerical parameter should at least be construed in lightof the number of reported significant digits and by applying ordinaryrounding techniques. Any specific value may vary by 20%.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The foregoingembodiments are therefore to be considered in all respects illustrativerather than limiting on the invention described herein. Scope of theinvention is thus indicated by the appended claims rather than by theforegoing description, and all changes which come within the meaning andrange of equivalency of the claims are intended to be embraced therein.

It will be appreciated by those skilled in the art that variousmodifications and changes may be made without departing from the scopeof the described technology. Such modifications and changes are intendedto fall within the scope of the embodiments that are described. It willalso be appreciated by those of skill in the art that features includedin one embodiment are interchangeable with other embodiments; and thatone or more features from a depicted embodiment can be included withother depicted embodiments in any combination.

What is claimed is:
 1. A reinforced composition for 3-D ink printing, said reinforced composition comprising: a UV-curable matrix material and a reinforcing agent, wherein the reinforcing agent is at least partially soluble in the UV-curable matrix material and wherein the solubility of the reinforcing agent in the UV-curable matrix material decreases as the UV-curable matrix material cures, whereby curing the UV-curable matrix material causes the reinforcing agent to form a phase-separate domain in the cured UV-curable matrix material.
 2. The reinforced composition of claim 1, wherein the UV-curable matrix material comprises an acrylate.
 3. The reinforced composition of claim 1, wherein the UV-curable matrix material comprises a thiol-ene.
 4. The reinforced composition of claim 1, wherein the UV-curable matrix material comprises a combination of acrylate and thiol-ene.
 5. The reinforced composition of claim 1, wherein reinforcing agent is selected based at least in part on a property of the UV-curable matrix material.
 6. The reinforced composition of claim 1, wherein the reinforcing agent has a higher glass-transition temperature than that of the UV-curable matrix material.
 7. The reinforced composition of claim 1, wherein the reinforcing agent is non-UV-curable.
 8. The reinforced composition of claim 1, wherein the reinforcing agent and UV-curable matrix material fail to react with each other.
 9. The reinforced composition of claim 1, wherein the cured reinforced composition is 30-300% has a tensile strength that is higher than that of the UV-curable matrix material in the absence of the reinforcing agent.
 10. The reinforced composition of claim 1, wherein the cured reinforced composition has a tensile strength that depends upon loading of the reinforcing agent within the reinforced composition.
 11. The reinforced composition of claim 1, wherein the reinforcing agent is present in an amount that is less than 20 wt %.
 12. The reinforced composition of claim 1, wherein the reinforcing agent is a polyvinylpyrrolidone.
 13. The reinforced composition of claim 1, wherein the reinforcing agent is a poly(methyl methacrylate).
 14. A method comprising jetting a layer of ink onto a structure that is being printed and causing dissolved reinforcing agent to precipitate after said layer of ink has been deposited.
 15. The method of claim 14, wherein causing said dissolved reinforcing agent to precipitate comprises curing said ink.
 16. The method of claim 15, wherein curing said ink comprises exposing said ink to UV.
 17. The method of claim 15, wherein curing said ink comprises exposing said ink to a temperature decrease. 